The calcium-binding photoproteins are present as a complex of an apoprotein and the peroxide of coelenterazine as a light-emitting substrate. The calcium-binding photoproteins emit a flash of light when bound to calcium ions.
Among the calcium-binding photoproteins including aequorin, obelin, clytin, mitrocomin, mineopsin and bervoin, aequorin is a well-characterized calcium-binding photoprotein, and its protein structure and the luminescence mechanism have been reported in detail (see, e.g., Inouye et al. (1985) Proc. Natl. Acad. Sci. USA 82, 3154-3158; and Head et al. (2000) Nature 405, 372-376). Due to its high sensitivity to calcium ions, aequorin is used to detect/quantify trace amounts of calcium ions, to measure changes in the concentration of intracellular calcium ions, and so on.
Clytin is a calcium-binding photoprotein isolated from the luminous jellyfish Clytia gregaria (see Inouye, S. and Tsuji, F. I. (1993) FEBS Lett. 315, 343-346; etc.). Clytin is present as a complex of apoclytin and the peroxide of coelenterazine as the light-emitting substrate. When bound to calcium ions, clytin emits a flash of light to produce coelenteramide, which is the oxidation product of coelenterazine, and carbon dioxide.
Herein, clytin can be classified into two groups, clytin-I and clytin-II (see, e.g., JPA 2008-22848; and Inouye, S (2008) J. Biochem. 143, 711-717). Of these two groups, clytin-II emits a flash of light when bound to calcium ions. The decay time of luminescence in clytin-II is a shorter than that of clytin-I.